Refrigeration system

ABSTRACT

Refrigeration systems are provided which include a first refrigeration system and a second refrigeration system that share a common heat exchanger. The heat exchanger includes a vessel that operates as a condenser for the second refrigeration system. A coil disposed in the vessel operates as an evaporator for the first refrigeration system. A method of operating a refrigeration system is also provided that includes exchanging energy between the refrigerant of the first refrigeration system and the refrigerant of the second refrigeration system.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application claims priority from and the benefit of U.S. Provisional Application No. 60/894,052, entitled SYSTEMS AND METHODS OF USING CO2 IN REFRIGERATION AND AIR CONDITIONING APPLICATIONS, filed Mar. 9, 2007, and U.S. Provisional Application No. 60/917,175, entitled SYSTEMS AND METHODS OF USING NATURAL REFRIGERANTS, filed May 10, 2007, which are hereby incorporated by reference.

BACKGROUND

The invention relates generally to refrigeration systems.

Many applications exist for refrigeration systems including residential, commercial, and industrial applications. For example, a commercial refrigeration system may be used to cool an enclosed space such as a cooler or a freezer. In another example, an industrial refrigeration system may be used to preserve, process, and store food. Very generally, refrigeration systems may include circulating a fluid through a closed loop between an evaporator where the fluid absorbs heat and a condenser where the fluid releases heat. The fluid flowing within the closed loop is generally formulated to undergo phase changes within the normal operating temperatures and pressures of the system so that considerable quantities of heat can be exchanged by virtue of the latent heat of vaporization of the fluid.

In some applications, two or more closed refrigeration loops may be interconnected to form a multistage refrigeration system. Multistage refrigeration systems (also referred to as cascade refrigeration systems or multi-pressure systems) may be used to provide cooling to multiple environments with different temperature requirements, such as a refrigerated case and a storage freezer. Multistage refrigeration systems also may be used to provide cooling temperatures that are lower that that attainable by a single-stage system, such as a vapor compression system.

SUMMARY

The present invention relates to a refrigeration system including a first refrigeration system configured to implement a refrigeration cycle with a first refrigerant, a second refrigeration system configured to implement a refrigeration cycle with a carbon dioxide refrigerant, and a heat exchanger common to the first and second refrigeration systems. The heat exchanger includes a vessel configured to operate as a condenser for the carbon dioxide refrigerant and a coil disposed in the vessel that is configured to operate as an evaporator for the first refrigerant.

The present invention also relates to a refrigeration system including a first stage system configured to implement a refrigeration cycle with a first refrigerant and a second stage system configured to implement a refrigeration cycle with a carbon dioxide refrigerant. The first stage system includes a compressor, a condenser, an expansion device, and an evaporator. The second stage system includes an expansion device, an evaporator, a receiver, and a pump. The first stage evaporator is disposed within the receiver.

The present invention further relates to a method for operating a refrigeration system that includes circulating a first refrigerant in a first refrigeration system and through a coil disposed in a vessel and circulating a carbon dioxide refrigerant in a second refrigeration system and through the vessel. The first refrigerant and the carbon dioxide refrigerant exchange energy with one another in the vessel to vaporize the first refrigerant in the coil and to condense the carbon dioxide refrigerant in the vessel.

DRAWINGS

FIG. 1 is an illustration of an exemplary embodiment of a commercial application incorporating a refrigeration system.

FIG. 2 is an illustration of an exemplary embodiment of an industrial application incorporating a refrigeration system.

FIG. 3 is a perspective view of an exemplary refrigeration system.

FIG. 4 is a front elevational view of the refrigeration system of FIG. 3.

FIG. 5 is a diagrammatical representation of an exemplary embodiment of a multistage refrigeration system.

FIG. 6 is a somewhat more detailed diagrammatical representation of the multistage refrigeration system of FIG. 5.

FIG. 7 is a diagrammatical representation of an exemplary embodiment of a multistage refrigeration system.

DETAILED DESCRIPTION

FIG. 1 illustrates an exemplary embodiment of a commercial application incorporating a refrigeration system 10 that provides both refrigeration and freezing capacity for a supermarket 12. Refrigeration system 10 includes a first stage system and a second stage system, each with their own closed refrigeration loop. Refrigerated display cases 14, which may contain refrigerated grocery items, such as milk and cheese, are maintained at a preset refrigeration temperature while freezer display cases 16, which may contain frozen grocery items, such as ice cream and pizza, are maintained at a preset freezer temperature. According to an exemplary embodiment, the refrigeration temperature may be between about 2 deg C. and about 7 deg 7 C, and the freezer temperature may be between about −20 deg C. and about −30 deg C. An evaporator 18, located in a freezer storage area 20, may be used to cool display cases 16 to the preset freezer temperature. An evaporator 20, located in a refrigerated storage area 24, may be used to cool display cases 14 to the preset refrigeration temperature. Evaporators 18 and 20 also may be used to cool freezer storage area 20 and refrigerated storage area 24 to the preset freezer temperature and the preset refrigeration temperature, respectively. Grocery items may be stored in storage areas 20 and 24 prior to placement within display cases 14 and 16. According to an exemplary embodiment, evaporators 18 and 20 are both part of the second stage system and are operated at different pressures to maintain the preset refrigeration temperature and the present freezer temperature.

FIG. 2 illustrates an exemplary embodiment of an industrial application incorporating refrigeration system 10 into an industrial building 26. Building 26 houses manufacturing operations, such as a food packaging and processing operations, that employ a plate freezer 28 to rapidly freeze flat products, such as pastries, fish fillets, and beef patties, as well as irregular-shaped items such as vegetables packaged in brick-shape containers. Horizontal or vertical plates 30 may be used to freeze and shape the products. Plates 30 are cooled to subfreezing temperatures by internally circulating refrigerant through thin channels within plates 30. Generally, the internal construction of plates 30 allows for a high rate of heat transfer between the products and plates 30. According to an exemplary embodiment, plates 30 are part of the second stage system are cooled to temperature of between about −20 deg C. and about −50 deg C.

FIG. 3 is a perspective view of an exemplary multistage refrigeration that includes a first stage system circulating a first fluid and a second stage system circulating a second fluid. The equipment used in the multistage refrigeration system includes, among other things, a heat exchanger 36, a compressor 38 and condenser 40 used in the first stage system, one or more compressors 48 used in the second stage system, a receiver 52, and a pump 54. Heat exchanger 36 is used by both the first stage system and the second stage system. For example, according to an exemplary embodiment, heat exchanger 36 may be used to vaporize liquid refrigerant circulating within the first stage system and condense vapor refrigerant circulating within the second stage system. Heat exchanger 36 may be a plate heat exchanger, a shell and tube heat exchanger, a plate and shell heat exchanger, or any other suitable type of heat exchanger. Compressors 38 and 48 receive vapor phase refrigerant flowing within each stage and reduce the volume available for the refrigerant, consequently, increasing the pressure and temperature of the refrigerant. The compressors may be any suitable compressor such as a screw compressor, reciprocating compressor, rotary compressor, swing link compressor, scroll compressor, or turbine compressor. According to an exemplary embodiment, compressor 38 compresses the vaporized refrigerant of the first stage system after it exits heat exchanger 36. Condenser 40 receives the compressed vapor and condenses the vapor into a liquid that after expansion is ready to enter heat exchanger 36 to begin the refrigeration cycle again. Receiver 52 may be used to store the liquid refrigerant within first stage system before the refrigerant enters heat exchanger 36.

The refrigerant circulating within the second stage system exits heat exchanger 36 as a liquid that flows to one or more evaporators (not shown). In the evaporators, the liquid phase refrigerant absorbs heat and vaporizes into a vapor phase refrigerant or a mixed vapor and liquid phase refrigerant. The refrigerant circulating within the evaporators absorbs heat from an external fluid allowing the evaporator to provide a cooled fluid, such as water or air, to an environment. According to an exemplary embodiment, the vapor phase refrigerant exiting one of the evaporators enters compressor 48 where it is compressed and ready to enter heat exchanger 36 to begin the refrigeration cycle again. Although only a few pieces of equipment are described, the multistage refrigeration may include numerous pieces of equipment such as multiple condensers, compressors, expansion devices, receivers, and evaporators, or combination thereof.

FIG. 4 depicts a front view of the multistage refrigeration system of FIG. 3 and shows heat exchanger 36, compressors 38 and 48, and receiver 52. FIG. 4 also depicts a pump 54 that may be used to circulate refrigerant within the second stage system.

FIG. 5 shows a multistage refrigeration system 56 that includes a first stage system 58 and a second stage system 60. The first and second stage systems each include a closed refrigeration loop for circulating a fluid within each system. According to an exemplary embodiment, a carbon dioxide refrigerant (R-744) may be circulated within the second stage system and ammonia (R-717) may be circulated within the first stage system. According to another exemplary embodiment, any fluid capable of performing as a refrigerant in a high temperature and pressure system, such as hydrocarbon based refrigerants or hydrofluorocarbon based refrigerants, may be circulated within the first stage system.

Heat exchanger 36 is utilized by both first and second stage systems 58 and 60. Heat exchanger 36 includes a receiver or vessel 62 configured to operate as a condenser for the second stage refrigerant and a coil 64 configured to operate as an evaporator for the first stage refrigerant. Coil 64 is disposed within vessel 62 and configured to circulate the first stage refrigerant within an interior volume 66. Although coil 64 is shown with a generally serpentine shape, other exemplary embodiments may include other suitable coil configurations. Furthermore, according to exemplary embodiments, the coil may be replaced by a plate, tube, or series thereof.

Liquid phase refrigerant 68 exits first stage system 58 and enters coil 64. As the refrigerant flows through coil 64, the refrigerant absorbs heat from the second stage refrigerant contained within interior volume 66. The heat causes the refrigerant within coil 64 to vaporize, creating vapor phase refrigerant 70 that exits heat exchanger 36 and enters first stage system 58. Within first stage system 58, the refrigerant completes the refrigeration cycle to change phase back to liquid phase refrigerant 68 that is ready to enter heat exchanger 36.

Vapor phase refrigerant 72 exits second stage system 60 and enters interior volume 66. Within interior volume 66, the vapor phase refrigerant transfers heat to the refrigerant in coil 64. The loss of heat causes the refrigerant within interior volume 66 to condense, creating liquid phase refrigerant 74 that exits heat exchanger 36 and enters second stage system 60. Within second stage system 60, the refrigerant completes the refrigeration cycle to change phase back to vapor phase refrigerant 72 that is ready to enter heat exchanger 36. A pool of liquid second stage refrigerant, in this case, carbon dioxide, will collect in the vessel with vapor phase carbon dioxide being resident above the liquid.

FIG. 6 shows the portions of the refrigeration cycles that occur within first stage system 58 and second stage system 60. Referring to first stage system 58, vapor phase refrigerant 70 exiting heat exchanger 36 enters compressor 38 where the refrigerant is compressed into a relatively high temperature and pressure vapor. The compressed vapor enters condenser 40 and transfers heat to a fluid, such as water from a cooling tower, flowing through condenser 40. The heat transfer causes the vapor phase refrigerant to condense into a liquid phase. The liquid phase refrigerant then flows into an expansion device 80 where the refrigerant expands, thereby reducing its temperature and pressure. According to an exemplary embodiment, the expansion device may be a thermal expansion valve; however, any suitable expansion device may be used. After exiting expansion device 80, the refrigerant enters coil 64 within heat exchanger 36 to begin the refrigeration cycle again.

Referring to second stage system 60, liquid phase refrigerant 74 exiting heat exchanger 36 enters pump 54 for circulation through second stage system 60. Pump 54 circulates the refrigerant to expansion valve 84 where the refrigerant expands to become a relatively low pressure and temperature liquid. The refrigerant then enters evaporator 82 and absorbs heat from a fluid, such as air or products in contact with the surface of evaporator 82. The heat transfer causes some, or all, of the liquid phase refrigerant to vaporize into a vapor phase refrigerant. The vapor phase refrigerant then enters interior volume 66 of heat exchanger 36 to begin the refrigeration cycle again. It should be noted that the equipment described in FIG. 5 is not intended to be limiting. Other equipment such as receivers, controllers, oil separators, fans, motors, and additional evaporators, condensers, and pumps, among other things may be used in the refrigeration cycles of first stage 58 and second stage 60 depending on factors such as the cooling capacity required, system sizes, and environmental temperatures.

FIG. 7 depicts an exemplary embodiment of a multistage refrigeration system that includes two evaporators 88 and 90 for cooling two environments at two different temperatures. Evaporators 88 and 90 each have a valve 92 or 94 located upstream to allow regulation of the pressure and temperature of the refrigerant entering each evaporator 88 and 90. Valves 92 and 94 allow each evaporator to provide cooling to a different temperature. For example, according to an exemplary embodiment, valve 94 may be a thermal expansion valve configured to reduce the pressure and temperature of the refrigerant that enters evaporator 90. In this embodiment, the refrigerant enters valve 94 as a relatively high pressure and temperature fluid and exits valve 94 at a reduced temperature and pressure. Within valve 94, a portion of the liquid refrigerant may be evaporated, which may cool the remaining liquid refrigerant. In this embodiment, valve 92 may be a solenoid valve configured to distribute the refrigerant into evaporator 88. Where operating pressures permit, the liquid refrigerant may flow through valve 92 and enter evaporator 88 at a temperature and pressure that are substantially the same as the temperature and pressure reigning in vessel 62. According to an exemplary embodiment, evaporator 90 receives refrigerant at a much lower pressure than the refrigerant entering evaporator 88. For example, according to an exemplary embodiment, evaporator 90 may receive refrigerant at a temperature of approximately −30 deg C. and a pressure of approximately 12 bar allowing evaporator 90 to provide cooling to a temperature close to −30 deg C. depending on, among other things, evaporator efficiency. In the same embodiment, evaporator 88 may receive refrigerant at a temperature of approximately −10 deg C. and a pressure of approximately 28 bar allowing evaporator 88 to provide cooling to a temperature close to −10 deg C. depending on, among other things, evaporator efficiency

First stage system 58 and second stage system 60 share heat exchanger 36, as described with respect to FIG. 5. Referring to second stage system 60, vapor phase refrigerant in interior volume 66 is condensed into a liquid phase refrigerant. According to an exemplary embodiment, the liquid phase refrigerant within interior volume 66 may collect in the bottom of vessel 62 to form a liquid reservoir. Liquid phase refrigerant 74 exiting heat exchanger 36 is circulated through second stage system 60 by pump 54. While any type of pump can be used, the pump is generally selected to have very low or no net positive suction head. An optional filter 96 may be included to receive a diverted portion of liquid refrigerant exiting pump 54. Filter 96 is positioned within a minimum flow line disposed lower than the refrigeration line that exits pump 54. Filter 96 filters the refrigerant to reduce sediment, small particles, and water entrained within the refrigerant (e.g., by absorption of water).

The refrigerant not diverted to filter 96 flows to a heat exchanger 98 where the refrigerant is cooled, and in exemplary embodiments may be sub-cooled. According to exemplary embodiments, heat exchanger 98 is a dual coil heat exchanger that includes a coil 100 that receives liquid phase refrigerant from pump 54 and a coil 102 that receives vapor phase refrigerant from evaporator 90. The liquid phase refrigerant exiting heat exchanger 98 flows to evaporator 88 or 90 through valve 92 or 94, respectively. Valves 92 and 94 allow regulation of the pressure and temperature of the refrigerant entering evaporators 88 and 90 to allow each evaporator 88 or 90 to provide cooling to a different temperature. Although evaporators 88 and 90 are used to cool environments to different temperatures, evaporators 88 and 90 also may be used to cool environments to similar temperatures.

The liquid refrigerant flowing through valve 92 enters evaporator 88 where the refrigerant absorbs heat from a fluid flowing through evaporator 88 causing some, or all, of the refrigerant to vaporize. The vapor phase refrigerant (or in some exemplary embodiments, the vapor and liquid phase refrigerant mixture) exits evaporator 88 and enters interior volume 66 of heat exchanger 36 where it condenses into a liquid to begin the refrigeration cycle again. Liquid phase refrigerant, which may enter interior volume 66 as part of a vapor phase and liquid phase mixture, may be collected in the bottom portion of vessel 62. According to exemplary embodiments, evaporator 88 is typically operated using refrigerant of a relatively high temperature (about 0 deg C. or less) and consequently high pressure, and, therefore, the refrigerant exiting evaporator 88 is able to flow directly back to heat exchanger 36 without being compressed. In these embodiments, the high pressure refrigerant returning directly to heat exchanger 36 may correspond to the pressure within vessel 62. However, in other exemplary embodiments, evaporator 88 may be operated using a lower temperature and pressure refrigerant and the refrigerant may enter a compressor prior to returning to heat exchanger 36.

The liquid phase refrigerant flowing through valve 94 enters evaporator 90 where the refrigerant absorbs heat causing the liquid to vaporize. According to exemplary embodiments, evaporator 90 is typically operated using refrigerant of a relatively low temperature (about −56 deg C. or more) and consequently low pressure, and, therefore, typically undergoes superheating and compression prior to returning to heat exchanger 36. The vapor phase refrigerant exiting evaporator 90 flows through coil 102 within heat exchanger 98 where the vapor phase refrigerant is heated, and in exemplary embodiment may by superheated. After exiting heat exchanger 98, the refrigerant enters compressor 48. Compressor 48 reduces the volume available for the vapor phase refrigerant, consequently, increasing the pressure and temperature of the vapor phase refrigerant. An optional heat exchanger 106 may receive the compressed vapor phase refrigerant exiting compressor 48. Heat exchanger 106 may be used to de-superheat the vapor phase refrigerant, thereby allowing some of the heat from the vapor refrigerant to be used to heat another environment or device. As the refrigerant flows within heat exchanger 106, a fan 108 draws air across heat exchanger 106. The fan may push or pull air across the heat exchanger. Heat transfers from the refrigerant to the air, producing heated air and cooling the refrigerant. Fan 108 is driven by a motor 110. According to exemplary embodiments, the fan may be replaced by a pump that circulates a fluid, such as water, through heat exchanger 106. According to an exemplary embodiment, the fluid may be freeze protected (e.g. by glycol or brine) to reduce the risk of cooling the fluid below its freezing point. After exiting heat exchanger 106, the refrigerant returns to interior volume 66 where it condenses into liquid to begin the refrigeration cycle again.

It may be noted that check valves or other arrangements may be provided around valves 92 and 94 to permit the return of refrigerant to the vessel, such as in the even flow to the evaporators is stopped, trapping liquid refrigerant that may vaporize in an otherwise confined volume.

Referring to first stage system 58, liquid phase refrigerant flowing within coil 64 is vaporized producing vapor phase refrigerant. The vapor phase refrigerant exiting heat exchanger 36 flows to a heat exchanger 112 where the refrigerant is heated, and in exemplary embodiments may be superheated. According to exemplary embodiments, heat exchanger 112 is a dual coil heat exchanger that includes a coil 114 that receives vapor phase refrigerant from heat exchanger 36 and a coil 116 that receives liquid phase refrigerant from receiver 52. The vapor phase refrigerant exiting heat exchanger 112 flows to compressor 38 where it is compressed to increase the temperature and pressure of the vapor phase refrigerant. The compressed vapor phase refrigerant enters heat exchanger 118 where the refrigerant is de-superheated before entering condenser 40. Within condenser 40, the vapor phase refrigerant transfers heat to a fluid, such as water, flowing through condenser 40. The heat transfer causes the vapor phase refrigerant to condense into a liquid. The liquid phase refrigerant exiting condenser 40 enters receiver 52 where it is stored prior to entering heat exchanger 112. According to an exemplary embodiment, receiver 52 may cause some of the uncondensed vapor phase refrigerant (if any is present) exiting condenser 40 to condense into a liquid. After exiting receiver 52, the refrigerant may flow through coil 116 of heat exchanger 112 where the refrigerant is cooled, and in an exemplary embodiment is sub-cooled. The refrigerant then enters expansion device 80 where the temperature and pressure is reduced before returning the refrigerant to coil 64 of heat exchanger 36 to begin the refrigeration cycle again. It may be noted that heat exchanger 116, which may be of a type sometimes referred to as a “suction line” heat exchanger, may not be used in all systems and with all refrigerants. For example, if the first refrigerant is ammonia, this heat exchanger may be eliminated, while it may be more useful with refrigerants such as hydrocarbons.

First stage system 58 also may include an optional closed loop 122 that is circulated through heat exchanger 118. Closed loop 122 allows heat absorbed from the vapor phase refrigerant within heat exchanger 118 to be transferred to a device 124. Device 124 may be any device that utilizes an input of heat. For example, device 124 may be a water heater or a furnace. A pump 126 circulates a fluid, such as water or any suitable refrigerant, within closed lop 122. According to an exemplary embodiment, the fluid may be freeze protected (e.g. by glycol or brine) to reduce the risk of cooling the fluid below its freezing point. The fluid flows through a coil 128 within device 124 and transfers heat to an interior volume 130. A fluid, such as air or water, may be circulated through interior volume 130 to transfer the heat absorbed to a suitable environment.

In accordance with an exemplary embodiment, the fluid circulated within second stage system 60 may be carbon dioxide. An optional coil 132 may be disposed within interior volume 66 to maintain the temperature within the interior volume in the case of a power failure or other equipment malfunction. Coil 132 may be used to decrease the temperature, and consequently, the pressure, within vessel 66 if the temperature and pressure exceed a specified value. For example, in the case of a power failure, coil 132 may be used to maintain the pressure within vessel 62 to prevent the carbon dioxide refrigerant from evaporating. A separate refrigeration unit (sometimes referred to as a stand still cooler) 134 may maintain the temperature of coil 132.

It should be noted that a number of additions and adaptations may be made to the vessel, as well as combinations of certain of the components and functions described above. For example, a level monitoring arrangement (e.g., electrical, mechanical or electromechanical) may be included for monitoring the level of liquid carbon dioxide in the vessel. In the event the level drops below a desired level, remedial measures may be taken, such as preventing operation of the pump of the second system. A signal or alarm may also be generated and conveyed to a system interface, controller, operator, or other recipient to signal such conditions.

Certain system components may be selected or placed in the vessel to further improve operation of the system, as well as to reduce the potential for loss of refrigerant and to improve packaging. For example, the pump utilized for the second system may be selected or designed to permit a reduced inlet head, thereby reducing the overall height of the system. Similarly, the pump may be placed in the vessel, thereby reducing or eliminating any need to otherwise cool a motor associated with the pump. The filter may also be placed in the vessel, where appropriate, to reduce external components.

A pressure limiting arrangement may also be included for limiting the pressure in the vessel to a high pressure, a low pressure, or both. In general, a pressure relief valve may form part of this arrangement to release vapor in the event of a high pressure situation (e.g., to avoid consequences that may be caused, for example, by washdown of an evaporator or system components by an operator). Low pressures may be avoided to avoid creation of ice that could affect operation of the pump.

It may be seen, then, that the vessel and its associated components may become one of the central elements of the combined (i.e., multistage) refrigeration system. By including as much functionality and as many components into this arrangement, then, the overall system may be made more robust and integrated.

The refrigeration systems may find application in a variety of applications. However, the systems are particularly well-suited to cooling two environments where each environment needs to be cooled to a different temperature. The systems are also particularly well-suited to cooling environments requiring temperatures within a range from about −54 deg C. to about 10 deg C. The systems may be used in the refrigeration systems described in P.C.T. Patent Application No. ______ (Attorney Docket No. 26427-0007) to Alexander Pachai et al., filed on Mar. 7, 2008, and P.C.T. Patent Application No. ______ (Attorney Docket No. 26427-0011) to Alexander Pachai et al., filed on Mar. 7, 2008, which are incorporated herein by reference in their entirety for all purposes. Other aspects that could be used in conjunction with the refrigeration systems are described in P.C.T. Patent Application No. ______ (Attorney Docket No. 26427-0003) to Holger Tychsen, filed on Mar. 7, 2008, P.C.T. Patent Application No. ______ (Attorney Docket No. 26427-0004) to Alexander Pachai et al., filed on Mar. 7, 2008, P.C.T. Patent Application No. ______ (Attorney Docket No. 26427-0005) to Alexander Pachai et al., filed on Mar. 7, 2008, P.C.T. Patent Application No. ______ (Attorney Docket No. 26427-0006) to Alexander Pachai et al., filed on Mar. 7, 2008, P.C.T. Patent Application No. ______ (Attorney Docket No. 26427-0008) to Alexander Pachai et al., filed on Mar. 7, 2008, P.C.T. Patent Application No. ______ (Attorney Docket No. 26427-0009) to Alexander Pachai et al., filed on Mar. 7, 2008, and P.C.T. Patent Application No. ______ (Attorney Docket No. 26427-0010) to Alexander Pachai et al., filed on Mar. 7, 2008, which are incorporated herein by reference in their entirety for all purposes.

While only certain features and embodiments of the invention have been illustrated and described herein, many modifications and changes may occur to those skilled in the art (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters (e.g., temperatures, pressures, etc.), mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter recited in the claims. The order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention. Furthermore, in an effort to provide a concise description of the exemplary embodiments, all features of an actual implementation may not have been described (i.e., those unrelated to the presently contemplated best mode of carrying out the invention, or those unrelated to enabling the claimed invention). It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation specific decisions may be made. Such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure, without undue experimentation. 

1. A refrigeration system comprising: a first refrigeration system configured to implement a refrigeration cycle with a first refrigerant, and a second refrigeration system configured to implement a refrigeration cycle with a carbon dioxide refrigerant; a heat exchanger common to the first and second refrigeration systems, the heat exchanger comprising a vessel configured to operate as a condenser for the carbon dioxide refrigerant and a coil disposed in the vessel and configured to operate as an evaporator for the first refrigerant.
 2. The system of claim 1, wherein the vessel is configured to receive vapor phase carbon dioxide refrigerant in an interior volume disposed about the coil.
 3. The system of claim 2, wherein the vessel is configured to retain a volume of liquid carbon dioxide refrigerant within the interior volume.
 4. The system of claim 1, comprising a carbon dioxide refrigeration system coupled to the vessel and configured to cool carbon dioxide refrigerant within the vessel during operation.
 5. The system of claim 1, wherein the second refrigeration system comprises an expansion device and an evaporator configured to provide cooling at an application via evaporation of liquid carbon dioxide.
 6. The system of claim 5, wherein vapor phase carbon dioxide refrigerant is directed from the evaporator to the interior volume of the vessel.
 7. The system of claim 1, wherein the second refrigeration system comprises a plurality of expansion devices associated with respective evaporators configured to provide cooling to different temperatures at a plurality of applications via evaporation of liquid carbon dioxide.
 8. The system of claim 7, wherein a circuit comprising one of the evaporators operating at a lower temperature than the other evaporator includes a compressor configured to compress vapor phase carbon dioxide refrigerant prior to returning the carbon dioxide refrigerant to the vessel.
 9. The system of claim 1, wherein the first refrigerant is an ammonia based refrigerant, a hydrocarbon based refrigerant, or a hydrofluorocarbon based refrigerant.
 10. A refrigeration system comprising: a first stage system comprising a compressor, a condenser, an expansion device and an evaporator, the first stage system configured to implement a refrigeration cycle with a first refrigerant; and a second stage system comprising an expansion device, an evaporator, a receiver and a pump, the second stage system configured to implement a refrigeration cycle with a carbon dioxide refrigerant; the first stage evaporator being disposed within the receiver.
 11. The system of claim 10 wherein the receiver is configured to operate as a condenser for the carbon dioxide refrigerant.
 12. The system of claim 10, wherein the receiver is configured to retain a volume of liquid carbon dioxide refrigerant within an interior volume in which the first stage evaporator is disposed.
 13. The system of claim 10, comprising a carbon dioxide refrigeration system coupled to the receiver and configured to cool carbon dioxide refrigerant within the vessel during operation.
 14. The system of claim 10, wherein the second refrigeration system comprises an expansion device and an evaporator configured to provide cooling at an application via evaporation of liquid carbon dioxide.
 15. The system of claim 14, wherein vapor phase carbon dioxide refrigerant is directed from the evaporator to an interior volume of the receiver.
 16. The system of claim 10 wherein the second refrigeration system comprises a plurality of expansion devices associated with respective evaporators configured to provide cooling to different temperatures at a plurality of applications via evaporation of liquid carbon dioxide.
 17. The system of claim 16, wherein a circuit comprising one of the evaporators operating at a lower temperature than the other evaporator includes a compressor configured to compress vapor phase carbon dioxide refrigerant prior to returning the carbon dioxide refrigerant to the receiver.
 18. A method for operating a refrigeration system, comprising: circulating a first refrigerant in a first refrigeration system and through a coil disposed in a vessel; and circulating a carbon dioxide refrigerant in a second refrigeration system and through the vessel; wherein the first refrigerant and the carbon dioxide refrigerant exchange energy with one another in the vessel to vaporize the first refrigerant in the coil and to condense the carbon dioxide refrigerant in the vessel.
 19. The method of claim 18, wherein the first refrigerant is an ammonia based refrigerant, a hydrocarbon based refrigerant, or a hydrofluorocarbon based refrigerant.
 20. The method of claim 18, comprising cooling two different applications at different temperatures via evaporation of the carbon dioxide refrigerant at two different pressures. 